In servicing major home appliances such as electric ovens and ranges, refrigerators and freezers, and the like, it is usually necessary to test the electric circuits in order to isolate the problem requiring servicing. The environment in which the serviceman must often work is not necessarily good and is often cramped with poor illumination, and there results a great tendency to burn out the electric meters used for testing the circuits. Combined voltage and ohmmeters have been proposed to obviate some of the present difficulties, but meters continue to be burned out at an excessively high rate. One of the major causes of damage to combined meters is that when the meter probes are connected into a live circuit the protection afforded the ohmmeter portion is often inadequately responsive to prevent the external voltage from damaging the ohmmeter. Also, the voltages which must be tested range from very low voltage such as five volts, to relatively high voltage such as three hundred volts, so that stepping volt meters are generally required, and often the serviceman must set the meter at approximately the correct voltage to be tested, but if the set range is far too low the meter will be damaged. In view of the often poor working conditions it may be difficult to read the meter, particularly the typical swing-type meter which is unnecessarily accurate and costly for use in servicing typical home appliances and usually require manual operation of stepping means for setting the meter in the correct range.
A combined volt-ohmmeter is shown in a U.S. Patent No. 3,536,998, in which a circuit for protecting the meter against excessive voltage is provided, but this circuit relies on relays which are too slow operating to provide dependable protection. Various protective systems are shown in relatively early patents including U.S. Pat. No. 3,023,326, and U.S. Pat. No. 3,024,388, both granted in 1962 and filed in the late fifties in which transistors are used for switching. A protective circuit utilizing a semiconductor is shown in U.S. Pat. No. 3,343,085, and a semiconductor switching device is shown in U.S. Pat. No. 3,066,258. A millivolt drop indicator utilizing transistors is shown in U.S. Pat. No. 3,392,332, and a diode limiter is shown in U.S. Pat. Nos. 2,964,650. 3,657,649, shows a relatively simple range switching system, and light emitting diode circuits are shown in U.S. Pat. Nos. 3,795,863, and 3,796,951.